Bacteria have many two-component signal-transduction systems (TCSs) that respond to specific environmental signals by altering the phosphorylated state of a response regulator. Although these systems are presumed to form an intricate signal network, the detailed mechanism of how they interact with each other remains largely unexplained. In this application, we will use Streptococcus mutans as a model organism to study a novel signaling pathway in bacteria. S. mutans is considered to be the primary etiological agent of dental caries and sometimes in infective endocarditis. This pathogen colonizes the oral cavity by formation of multispecies biofilm and has developed a variety of mechanisms to adapt and to flourish in the hostile environment of the oral cavity. We found that one particular TCS, LiaFSR, which encodes an extra protein (LiaF) in addition to the sensor kinase (LiaS) and the response regulator (LiaR), regulates the expression of gbpC that encodes a glucan-binding-protein necessary for biofilm formation and the onset of endocarditis. We also found that LiaFSR is necessary for the production of mutacin, a bacteriocin, to suppress the growth of other competitor bacteria present in the biofilm community. A recent microarray study revealed that 174 genes, ~9% of the genome, are regulated by this TCS. In Bacillus and Staphylococcus, this TCS is involved in sensing cell wall damage caused by various antibiotics, particularly those antibiotics that interfere with the lipid- II cycle. However, the signals sensedby this TCS and the molecular mechanism of signal transduction in streptococci are yet to be identified. A puzzling question is how LiaS and LiaR regulate target gene expression in S. mutans, since inactivation of LiaR does not produce, in many cases, any noticeable phenotypes. In this application, based on our preliminary results, we propose a model to explain how LiaS and LiaR may participate in signal transduction. We also propose a possible role for LiaF in LiaSR mediated gene regulation. Specific Aims 1 and 2 are designed to test our hypothesis to determine the mechanisms by which LiaS and LiaF participate in signal transduction. The goal of Aim 3 is to understand the molecular mechanism of DNA binding by LiaR. Upon completion, we hope to determine the cellular role of the LiaFSR that may lead to greater means of controlling this pathogen. This investigation will also promote our overall understanding of the molecular mechanisms of gene regulation and signal transduction in S. mutans and other related pathogens such as group A- and group B- streptococcus.